This invention relates to a magneto ignition system for internal combustion engines which use semiconductor or solid-state devices to perform switching functions somewhat analogous to the functions performed by prior art breaker points.
There are prior art solid-state magneto ignition control systems wherein a permanent magnet traverses a magnetic core or stator synchronously with engine rotation. A low voltage primary winding and a high voltage secondary winding on the stator constitute a step-up transformer or ignition coil and the secondary winding is connected to the spray plug. There is a semiconductor power switching device, such as a transistor or Darlington transistor pair connected in series with the primary coil. As the field of the rotating magnet becomes coupled with the stator an increasing essentially sinusoidal voltage is developed in the primary winding and at some small positive voltage level the power switching device turns on and current through the primary winding rapidly increases to a value that is limited only by the total impedance of the primary winding and switching circuitry. A control transistor is typically used in the prior designs in conjunction with the power switching device. The control transistor responds to sensing that the voltage across the primary winding has reached a specific level by becoming conductive to thereby cut off the driving current for the semiconductor power switching device and the latter switches to a nonconductive state abruptly. The abrupt interruption of current flow through the primary winding causes the magnetic field to collapse so as to induce a high voltage in the secondary winding to thereby produce a spark plug discharge.
When a transistor is used for the voltage sensing or control device, the power switching device will remain off only as long as the primary winding voltage remains higher than the predetermined voltage at which switching first occurred. When the primary winding voltage falls below that level, the power switching device will turn on again and a new operating cycle will begin if enough energy is still available in the rotating magnet and magneto stator. This can result, undesirably, in an ignition voltage waveform having a series of short duration pulses when the control transistor technique is applied to some magneto ignition coils.
In many instances, engine designers prefer a single ignition pulse of relatively long duration rather than a series of short duration pulses as commonly occur in ignition switching circuits that use an ordinary transistor for voltage sensing or control of the power switching device. Known systems that use an SCR for switching control have a capability of providing a longer duration ignition spark, since once an SCR has switched on it will remain on, and the power switching device will remain off, as long as current is sustained through the SCR. The SCR desirably will remain on and the power switching device off until generated ignition coil primary winding voltage reaches a much lower level than for ignition control circuits where a transistor is used for controlling the power switching device.